1. Field of the Invention
The present invention is directed generally to a method and apparatus to hold integrated circuit chips on, and remove the chips from, a cutting chuck without the use of an adhesive or a wafer frame. More particularly, the invention is directed to a method and apparatus using differential pressure to both hold the chips on the cutting chuck and to remove the chips from the cutting chuck.
2. Description of the Background
Integrated circuits have touched almost every aspect of society, such as children's games and toys, engine computers in automobiles, personal computers in homes and offices, and controllers in industrial processes. Better ways to fabricate integrated circuits are constantly being sought.
Integrated circuits are fabricated on semiconductor wafers, and each wafer typically contains between 50 and 1,000 individual integrated circuits, depending on the size of the wafer and the size of the individual integrated circuits. Between the integrated circuits are spaces, known as "street indices", which separate the individual integrated circuits on the wafer. Street indices are as small as possible, and are typically four mil to six mil wide. In a process known as "dicing", wafers are cut along the street indices to form separate integrated circuits, known as "dice". A street index which has been cut is known as a "street". When the dicing process is completed, the streets form a grid which defines the dice cut from the wafer.
The dicing process is performed with a cutting assembly having a circular cutting blade. The design and use of the cutting assembly and cutting blade are well known in the prior art, and such devices may be obtained from Disco Hi Tec America, Inc., located in Santa Clara, Calif. The cutting blades are about one mil thick and spin at speeds between 30,000 and 60,000 revolutions per minute. Cutting blades are often nickel-plated with a diamond grit cutting edge to insure smooth, clean cuts, with minimal fraying and splintering.
Wafers are placed on a smooth, level surface, known as a "cutting chuck", where they are diced by the cutting blade. During the dicing process, the cutting blade will occasionally protrude below a wafer and into the underlying cutting chuck. The contact between the cutting blade and cutting chuck accelerates the wear on the cutting blade, and often breaks the cutting blade and results in damage to the cutting chuck.
It is well known in the prior art to use a wafer frame and adhesive tape to maintain dice in place during the dicing process. The wafer frame is generally flat and defines an opening which is larger than the wafer. The adhesive tape is attached to the wafer frame and stretched across the opening. A wafer is secured to the adhesive tape within the opening, and the frame is secured, for example by a vacuum, to the cutting chuck for dicing. After the dice have been cut, the frame, along with the adhesive tape and the dice, are removed from the cutting chuck. The dice are separated from the adhesive tape, the adhesive tape is removed from the frame, and the frame is reused. The adhesive tape is known as "sticky back" and is usually a polymer-based film, such as polyvinyl chloride ("PVC"), with an adhesive coating on one side. The adhesive tape is usually about 3 mils thick. The dice stick to the adhesive, so that when the wafer is cut the dice remain in place on the cutting chuck and are not scattered. Because a cutting blade extends slightly below the wafer, the cutting blade is exposed to the adhesive tape. The adhesive binds to the cutting blade, causing accelerated blade wear and "gumming-up" the cutting blade. A gummed-up cutting blade reduces the effectiveness of the cutting blade, increases friction between the cutting blade and the wafer, and increases the tendency of the cutting blade to bind and break. Heat is generated from friction between the cutting blade and both the wafer and the sticky-back. The faster the cutting blade is moved through the wafer, the more heat is generated, and that heat is increased when the cutting blade is gummed-up. In addition, the risk of the cutting blade binding increases as the temperature of the cutting blade increases. Furthermore, the integrated circuits may be damaged by the heat. As a result, the heat generated by the dicing process, and all of the undesirable side effects of the heat, limits the rate at which the cutting blade can be moved across a wafer. As the rate of the dicing processes decreases, the amount of time required to dice a wafer increases.
The accelerated wear and damage caused to cutting blades from contact with the chuck and the adhesive requires that they be replaced after dicing only about five or six wafers. Worn cutting blades lack the sharpness to cleanly cut a wafer, and cutting blades exposed to adhesives have rough sides and an irregular cutting surface formed from hardened adhesive picked up during previous cuts of a wafer. The continued use of a worn cutting blade may result in damaged or totally destroyed wafers caused by a cutting blade breaking and spraying debris across the wafer. Replacing cutting blades is expensive not only in terms of the costs of the cutting blade, but also in terms of down time of the dicing process and interruption of the fabrication process while an old cutting blade is being removed and a new cutting blade is being installed.
After the dicing process is completed, the dice need to be tested and the good dice separated from the bad. The wafer, frame, along with the associated adhesive tape and dice, may be removed from the chuck and placed at a testing station, or the testing may be done on the cutting chuck. During testing, each die is individually tested to determine whether it is functional, and if so, whether it meets the specifications set for the chip. Information regarding which die are good and which are bad, along with other test data, is stored in a memory device. Since there are almost always some bad dice, a die pick is used to selectively remove only the good dice for further processing. The bad dice remaining on the adhesive tape are discarded.
The construction of die picks are well known in the prior art. Die picks include a moveable suction device which develops a pressure differential relative to the ambient pressure around the die pick. The suction device has a single, small suction opening or port, so that it engages only one die at a time. The suction device is maneuvered against a single good die and a suction is used to engage the die. Once engaged, the die pick moves the die to another station and releases it for further transport or processing.
As stated above, wafers often yield as many as 1,000 dice. As a result, a large number of dice must be removed by the die pick. Because die picks move only a single die at a time, they are a bottleneck in the process of fabricating semiconductor chips. For example, a conventional die pick typically requires between one half of a second and one second to remove one die from a group of dice and place it on another processing station. If five hundred dice need to be moved by the die pick, a delay of between 250 and 500 seconds occurs while the dice are being removed.
Thus, the need exists for an improved method and handling assembly which reduces the amount of time required to separate good dice from bad dice. In particular, the need exists for a handling assembly which can remove all of the good dice in a single operation.
Furthermore, the need exists for an improved cutting chuck which reduces the amount of wear and damage to a cutting blade. In particular, the need exists for a cutting chuck which does not interfere with a cutting blade during dicing, and which prevents contact between a cutting blade and adhesives currently used to secure a wafer during dicing.